An Orthogonal Frequency Division Multiplexing (OFDM) technology is essentially a multicarrier modulation communication technology and also one of core technologies in B3G and 4G mobile communications. In a frequency domain, a multipath channel for the OFDM presents a feature of frequency selectivity decline; in order number to overcome the decline, the channel is classified into multiple sub-channels in the frequency domain, each sub-channel having an approximately flat frequency spectrum and being orthogonal with one another, therefore the frequency spectrums of the sub-channels are allowed to be overlapped with one another and the frequency spectrum resources can be maximally utilized.
A Multiple Input Multiple Output (MIMO) technology can increase the capacity of a system, improve transmission performance and well integrate with the OFDM and other physical layer technologies, thereby becoming a key technology of a B3G and 4G mobile communication system. With the intensive study of the MIMO technology, at present, it is generally acknowledged that closed-loop MIMO (adaptive MIMO) has more performance gains than open-loop MIMO, mainly because a transmitting side can pre-process a transmitted signal when acquiring all or partial channel information and disturbance information, thereby adapting to the change of the channel and disturbance. Such gains are mainly present in Multiuser MIMO (MU-MIMO) and Cooperation Multi-Point (CoMP) transmission modes. In these two modes, disturbance is very obvious, so it is more necessary to restrain the disturbance by pre-processing a transmitted signal at the transmitting side.
In an MIMO communication system, a reference signal is designed by two methods:
In one method, the reference signal is defined on a physical antenna port, and then a receiving end is informed via a signalling of a method adopted for pre-processing transmitted data (generally in a linear pre-coding manner). Such reference signals are called Common Reference Signals (CRS) because the designed signals are distributed on the whole system bandwidth and all receiving ends can perform channel estimation from themselves to an antenna port through the CRSs. Such CRSs can serve as measurement reference signals as well as demodulation data reference signals and occupy more resources because of their distribution on the whole bandwidth, therefore, the channel estimation is very accurate. However, for the CRSs, it is also necessary for the transmitting side to additionally inform the receiving end of the method for pre-processing transmitted data and the overhead is large when there are a large number of transmitting antennas, so the CRSs are not very suitable. MIMO transmission modes (except transmission mode 7) in a Long Term Evolution (LTE) system adopt such reference signals.
In the other method, the reference signal itself is pre-processed in the same way as data, and is a dedicated signal rather than a common reference signal and only distributed on the same bandwidth resources with the data because such reference signal may be pre-processed in different manners for each receiving end. Besides, being pre-processed in the same way as data, the reference signal cannot serve as a measurement reference signal (which is not within the discussion scope of the patent) but only serve as a demodulation data reference signal for coherent demodulation of data, so the reference signal of the second type is called a dedicated demodulation data reference signal. The dedicated demodulation data reference signal here should be defined on a Layer (the transmitted data is also carried on the layer for transmission) because the data and the reference signal are pre-processed in the same way. The pre-processing (pre-coding) refers to mapping from a physical antenna to a layer, each layer corresponding to different pre-processing of multiple physical antennas. At present, such dedicated demodulation data reference signal is used for an Enhanced Long Term Evolution (LTE-A) system.
In the LTE-A system, it is not only basically determined to adopt dedicated demodulation data reference signals but also stipulated that the dedicated demodulation data reference signals in each Resource Block (RB) has a total overhead of 12 Resource Elements (REs) when there are less than or equal to 2 layers and a total overhead of less than 24 REs when there are 3 to 8 layers.
Although the overhead of the dedicated demodulation data reference signals has been basically determined in the LTE-A system and other systems, the locations of the REs occupied by the signals in an RB and multiplexing methods among different layers are not determined at present.